As a result of the steadily increasing demand and cost of fossil fuels, and environmental factors related to the use of fossil fuels, the development of alternative and renewable energy sources is on the rise.
One area of particular attention is the conversion of wind energy into a useful form whether it be in the form of mechanical energy to drive a mechanical system such as a pump or in the form of electrical energy. Windmills have been used throughout the ages in converting wind energy into a useful form. There are two basic types of windmills or wind turbines, the horizontal axis windmill and the vertical axis wind turbine.
Vertical axis wind turbines have inherent advantages of stability due to gyroscopic action of the rotor, simplicity of design due to the avoidance of yaw mechanisms and blade controls, and strength of construction. However, the fact that the blades of the rotor are exposed to the force of the wind during only one-half of each cycle and then must be shielded from the wind to prevent creation of back pressure during the remaining half of each cycle has been a major problem. A variety of structural changes have been proposed in an effort to avoid or minimize formation of back pressure on the sails during their return sweep. These efforts have not been successful in overcoming the problems associated with the prior known vertical-axis wind machines. As a consequence, vertical-axis machines have not been commercially attractive and have not achieved substantial acceptance in competition with the horizontal-axis windmills.
The windmill construction which has been most commonly utilized for the generation of electricity is a plural-bladed propeller positioned vertically for rotation about a horizontal axis. This type of construction has been widely used because, when positioned into the wind, the entire surface of each blade of the propeller is exposed to the full force of the moving air. The commercial windmill industry has developed around the horizontal-axis construction and the aerodynamic principles and knowledge discovered in connection with atmospheric flight. Accordingly, it has become common practice to design such machines for the atmospheric/wind conditions of specific locations by varying the number and/or dimensions of the blades employed. The fewer the propeller blades, the more efficient the machines become at high wind speeds but the less efficient they are at lower wind speeds.
Because the blades of horizontal-axis windmills are coupled indirectly to an electric generator which is effective only at a constant design speed, and because the blades themselves become unsafe at high speeds, the horizontal-axis windmills have been capable of utilizing only a small percentage of the theoretically-available power in the wind. The multi-blade windmills have high starting torque at low wind speeds, harvesting up to 30% of the kinetic energy from the wind but become very inefficient at high wind speeds. The Dutch 4-blade machines, for instance, utilize only about 16% of the winds' kinetic energy. The most common and efficient windmills today are of the two and three blade types designed for high tip speed operation. These machines harvest roughly 42% of the theoretical 59.2% kinetic energy from the wind. Such windmills operate within a narrow window or range of wind velocities defined by a cut-in wind speed of 3-5 mps (meters/sec.) and a cut-out wind speed of about 25 mps. To maintain a near constant level of torque to drive the generator has required either: complex controls, in the case of pitch control, or intricate blade designs, in the case of stall control, both of which are expensive to build and maintain. In addition, such wind machines require yaw mechanisms with motors, gearboxes, cable twist counters, etc. to keep the machine yawed against the wind. These requirements have combined to make the horizontal-axis windmills economically unattractive except in areas where alternative forms of electricity generation are not readily available.
Today's windmill designs also have other drawbacks. They have problems with gyroscopic vibration when the machine veers with changing wind direction. They are vulnerable to high bending moments at the base or root of the blades as each blade passes by or into the wind-shade of the supporting mast as well as when being braked during tempest conditions. These bending moments lead to frequent blade replacements and high maintenance costs. Because of their massive structures, these machines, of necessity, are remotely located miles from the area of power usage, thus necessitating construction of expensive power grids to transport the energy produced to the point of consumption, (generally large cities). Consequently, an approximate eight to ten percent of the power generated never reaches its destination due to line and transformer losses. Lastly, because of opposition from environmentalists with regard to the esthetics in natural settings as well as prohibition from municipal regulating authorities due to safety hazards associated with these large-prop machines in populated areas, many areas which would be ideal for generating wind energy, such as atop large buildings, are simply off-limits due to opposing design constraints.
Accordingly, there is a need for a vertical axis wind turbine of an improved and simplified construction that can be utilized both in urban and rural settings, that does not depend upon wind direction or wind velocity for optimal energy production, and that utilizes the back pressure during the remaining half of a rotation cycle to apply a breaking force to the rotor of the turbine to prevent overspeed, while overcoming the drawbacks of prior vertical axis wind turbines.